Hydraulic fracturing of subterranean formations (hereinafter referred to simply as “fracturing”) often incorporates the use of viscous solutions of water-soluble polymers. These water-soluble polymer solutions are used to translate hydraulic force to the subterranean formation in order to initiate a fracture in a subterranean formation, propagate the fracture, and to facilitate the transport and distribution of particulate proppants within the fracture to sustain a highly conductive pathway with which to produce oil and gas. Water-soluble polymers historically employed in hydraulically fracturing subterranean formations are guar and derivatized guars, derivatized celluloses, and polyacrylamides. Elements (hereinafter referred to as “metals”) historically used to crosslink the heretofore mentioned water-soluble polymers have been water-soluble forms of aluminum, antimony, boron, chromium, titanium, and zirconium. The crosslinking metals most commonly used today are boron and zirconium as crosslinkers for guar, hydroxypropyl guar, carboxymethyl guar, and carboxymethyl hydroxypropyl guar.
Of the Group IVB metals, titanium has been used to form crosslinked hydraulic fracturing fluids since about the mid-1970's and zirconium has been used in these same applications since about the early 1980's. Hafnium has been evaluated for use as a crosslinker in fracturing applications, but has yet to find a commercial following.
Generally speaking, Group IVB metal crosslinkers manufactured for use in fracturing applications have been either alcohol-based alkoxides or aqueous-based carboxylates. While a number of such crosslinkers have been used “as-is,” meaning “as-supplied” by the manufacturer of the crosslinker, both alcohol-based alkoxide and aqueous-based carboxylate Group IVB metal crosslinkers have also been blended with alkanolamines, alkylamines, beta-diketones, polyols, carboxylic acids and carboxylic acid salts for subsequent use in fracturing applications. The addition of such blended additives is used to control the rate of crosslinking, the temperature at which crosslinking begins, and/or the rheological stability of the crosslinked fracturing fluid during the well treatment. Low molecular weight alcohols (i.e., methanol, isopropanol, n-propanol, etc.) have been blended with both alcohol-based alkoxide and aqueous-based carboxylate Group IVB metal crosslinkers as diluents and to lower the freeze point of the crosslinker.
Of the zirconium alcohol-based crosslinkers, probably the earliest and most widely used product is n-propyl zirconate, an alkoxide hereinafter referred to as “NPZ.” Williams (U.S. Pat. No. 4,534,870) describes crosslinker compositions for well treatment applications incorporating NPZ and n-butyl zirconate (NBZ). Williams teaches the use of NPZ and NBZ alkoxides with triethanolamine and n-propyl and n-butyl alcohol, respectively, to form crosslinker compositions. The Williams' crosslinkers are flammable by virtue of the alcohol present in the NPZ/NBZ alkoxides as manufactured, and by the additional alcohol added to render the crosslinker composition suitable for use in well-treatment applications.
Subsequent improvements to the Williams crosslinker (for instance, U.S. Pat. Nos. 4,686,052, 4,683,068, and 4,693,254) include compositions with different ratios of triethanolamine to zirconium, and to the addition of water. These changes in the crosslinker composition as used in fracturing applications relate only to improvements in crosslinked fluid performance. They do not alter the flammable nature of the crosslinker, or vary their characterization of unused or contaminated portions as a hazardous waste because of the inherent presence of the alcohols. Alcohol-based Group IVB metal alkoxide crosslinkers are inherently less stable than their aqueous-based counterparts. Improved stability in the aqueous-based Group IVB metal carboxylate crosslinkers is believed to be due, in part, to the presence of the carboxylic acid(s) which serve to chelate the metal, whereas the absence of such stabilizing ligands in the alcohol-based Group IVB metal alkoxides make them extremely susceptible to oxo-bonding and subsequent precipitation in the presence of water or hydroxides. Precipitation of the metal renders the crosslinker unusable, hence the greater ease by which a hazardous waste might be generated from an alcohol-based Group IVB metal alkoxide complex than from the aqueous-based Group IVB metal carboxylic acid chelate.
Aqueous-based zirconium crosslinkers (for instance, U.S. Pat. Nos. 5,182,408, 5,466,846, and 5,798,320) provide for non-flammable and subsequently more environmentally-friendly crosslinkers for fracturing applications. Most of the aqueous-based zirconium carboxylate crosslinkers finding commercial application were alpha-hydroxycarboxylic acid derivatives of zirconium carbonates or zirconium sulfates. Many of the crosslinkers used in fracturing oil and gas wells were blends of zirconium carboxylates with alkanolamines, alkylamines, beta-diketones and/or polyols for the very same reasons these compounds were added to the alcohol-based crosslinkers. These crosslinkers were also diluted with water, glycols, and/or alcohols to improve field use and, in the case of the glycols and alcohols, to lower the freeze point of the crosslinker for use in sub-zero operating conditions. Aqueous-based zirconium crosslinkers have been used in polymer solutions made with both fresh-water and sea-water. They have been found suitable as crosslinking agents for guar, derivatized guar, derivatized cellulosics, and polyacrylamides. They have been used in wells with bottom-hole temperatures as low as about 85° F. and as high as about 450° F. These aqueous-based zirconium crosslinkers have been used across a very-wide pH range including, among others; use in concentrated hydrochloric acid solutions, use in carbon-dioxide foams, and use in fracturing fluids to pH 12+. Aqueous-based zirconium crosslinkers comprise a significant percentage of all zirconium crosslinkers employed in fracturing applications. Nevertheless, there is a significant quantity of alcohol-based zirconium alkoxide crosslinkers still finding use where the rheological performance obtained with alcohol-based Group IVB metal alkoxide crosslinkers is preferred. As such, there is a recognized need for Group IVB metal alkoxide crosslinkers that are non-aqueous and yet non-flammable for those fracturing applications where the rheological performance obtained with alcohol-based Group IVB metal alkoxide crosslinkers is preferred. Further, there is a desire for such crosslinkers to be of such a nature that unused portions thereof (or portions otherwise rendered unusable) do not contribute to the volume of hazardous wastes.